Methanol is produced commercially from synthesis gas comprising hydrogen, carbon monoxide, and carbon dioxide by contacting the synthesis gas with a solid methanol synthesis catalyst in one or more gas phase synthesis reactors. Most of the world's methanol is produced by this reaction route utilizing the well-known Lurgi and ICI methanol synthesis processes. An improved methanol synthesis process which utilizes powdered catalyst mixed with an inert liquid, known as the liquid phase methanol process, is disclosed in U.S. Pat. Nos. 3,888,896, 4,031,123, and 4,567,204. This process operates more efficiently than the gas phase process because the heat of reaction is absorbed by the inert liquid thus allowing closer temperature control in the reactor; heat is removed from the inert liquid in a separate cooling step. As a result of closer temperature control, a higher per pass conversion to methanol can be achieved with the liquid phase process than with conventional gas phase reactor systems. The liquid phase methanol process can utilize a much wider range of synthesis gas compositions than gas phase processes, and is particularly useful for CO-rich synthesis gas such as that produced by the coal gasification processes. Further improvement in operating efficiency and methanol yield in the liquid phase process can be realized by using multiple stages as disclosed in U.S. Pat. No. 4,766,154, the specification and drawings of which are incorporated herein by reference. This process utilizes a two-stage slurry reactor system which is operated such that a specific reaction mechanism occurs in each reactor; in the first stage reactor, the operation is controlled to favor the hydrogenation of carbon monoxide, while in the second stage reactor the operation is controlled to favor the hydrogenation of carbon dioxide. The staged process results in substantial increases in methanol yield over a single-stage liquid phase process. The staged process comprises feeding synthesis gas containing hydrogen, carbon monoxide, and carbon dioxide into the first stage reactor, withdrawing a stream containing methanol and unreacted synthesis gas components, cooling the stream and recovering methanol product therefrom, passing the unreacted synthesis gas components into the second stage reactor, and recovering additional methanol product from the second stage reactor effluent stream. Unreacted synthesis gas components are recycled to the inlet of the second stage reactor. Heat is removed from the inert liquid of each reactor by withdrawing a portion of the inert liquid as a slurry, cooling the slurry in an external heat exchanger, and pumping the cooled slurry back into the reactor.
U.S. Pat. No. 2,467,802 discloses a staged, gas-phase fluidized bed process for converting synthesis gas containing hydrogen and carbon oxides into hydrocarbons and oxygenated organic compounds. The process comprises passing synthesis gas upward through a series of fluidized catalyst beds, forming product components in each of the beds, and withdrawing product and unreacted synthesis gas from the last stage reactor. The volume and diameter of each reactor stage decreases in the direction of gas flow. Fresh catalyst is introduced into the last stage reactor and is partially spent therein; the catalyst then flows through one or more additional beds in series in a countercurrent direction to the synthesis gas flow, and finally spent catalyst is removed from the first stage reactor. Preferably, reaction products are removed from the gas stream between each stage and the unreacted synthesis gas is passed into the next stage. Heat is removed from each stage by a cooling coil installed in each reactor. As a result of the countercurrent flow of gas and catalyst, and the changing of the sizes of the reactor stages in the direction of gas flow, the optimum conditions of pressure, temperature, and gas velocities may be approximately the same in all reactor stages.
U.S. Pat. No. 2,852,350 discloses a reactor for the production of hydrocarbons from hydrogen and carbon monoxide in a slurry reactor in accordance with the Fisher-Tropsch process. The reactor consists of a pressure vessel containing a multiple number of internal vertical bundles of cooling tubes wherein the bundles are arranged such that the effective number of tubes and thus the effective heat transfer surface area decreases in the direction of gas flow. Gas flows upward through a catalyst slurry on the shell side of the tubes while coolant flows within the tubes.
A medium-load power generating plant with an integrated coal gasification plant is disclosed in U.S. Pat. No. 4,608,818. During periods of low power requirements excess synthesis gas is converted to methanol in parallel gas-phase methanol synthesis reactors. One of these methanol synthesis reactors is operated in a once-through mode and the unreacted synthesis gas is returned to the plant synthesis gas handling system. In the remaining methanol synthesis reactors the unreacted synthesis gas is recycled as reactor feed.
U.S. Pat. No. 4,665,688 discloses a power generating plant with an integrated coal gasification plant which produces synthesis gas, one portion of which is fired in a gas turbine power generation system and the remainder of which is used to produce methanol and other chemical products such as acetic acid. Unreacted synthesis gas from the methanol and acetic acid reactors is utilized for gas turbine fuel.
U.S. Pat. No. 4,946,477, the specification and drawings of which are incorporated herein by reference, discloses an improved integrated coal gasification combined cycle power generation process including once-through methanol production from the synthesis gas provided by the gasifier. Unreacted synthesis gas is utilized in a gas turbine generator to produce power. The improvement comprises utilizing a liquid phase methanol synthesis reaction system, a portion of the unreacted synthesis gas from which is separated into a hydrogen-rich stream and a CO-rich stream; the CO-rich stream is used as gas turbine fuel. The remainder of the unreacted synthesis gas is combined with the hydrogen-rich stream, CO.sub.2 is removed from the combined stream, and the resulting stream feeds a gas-phase methanol reactor to produce additional methanol. Unreacted synthesis gas from this reactor is used as gas turbine fuel.
The liquid phase reaction process is an efficient method for the production of methanol from synthesis gas, and particularly CO-rich synthesis gas. When synthesis gas is obtained by gasifying coal, the composition of the gas may vary over time due to changes in coal feedstock properties and gasifier operating conditions. In particular, the concentration of certain compounds which reduce the activity of the catalyst may change with time, thus in turn affecting the optimum operation of the process. Changes in the relative amounts of hydrogen and carbon oxides in the synthesis gas further affect the optimum operation of the process. The present invention as disclosed and claimed below addresses these problems and teaches an improved method of operating liquid phase methanol reactors and integrating the reactors with a coal gasification combined cycle power generation system.